学用 ASP.Net 之 "字符串" (5): StringBuilder 类

StringBuilder 类存在的理由是: 在对字符串增删改时, 比 string 类更快; 更快的原因是无须像 string 一样反复地释放和重建.
但它会在需要时重新分配内存, 如果把需要的内存预制好, 它会更快.

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StringBuilder 的属性与方法:

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/* 属性 */
Capacity;         // 容量; 可读写, 也可在构建时设置, 但改写时使用 EnsureCapacity() 更保险
Length;           // 长度; 可读写, 写入 0 时, 同 Clear(); Length <= Capacity <= MaxCapacity
MaxCapacity;      // 最大容量; 只读, 只能在构建时设置, 超出则异常

/* 方法 */
Append();         // 追加; 为接受不同类型的数据, 它有很多重载
AppendFormat();   // 按格式追加
AppendLine();     // 追加换行符
Clear();          // 取消; 之后 Length = 0; 但 Capacity、MaxCapacity 不变
CopyTo();         // 复制指定部分到 Char[]
EnsureCapacity(); // 设置 Capacity
Insert();         // 插入
Remove();         // 移除
Replace();        // 替换
ToString();       // 输出文本, 可同时截取

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构造函数的 6 种重载:

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using System.Text; // StringBuilder 所属的命名空间 

protected void Button1_Click(object sender, EventArgs e)
{
    string str = "";

    /* 无参时, Capacity 默认 16 */
    StringBuilder sb1 = new StringBuilder();
    str += sb1.Capacity.ToString("Capacity: #\n"); //Capacity: 16

    /* 构建时指定 Capacity 大小 */
    StringBuilder sb2 = new StringBuilder(11);
    str += sb2.Capacity.ToString("Capacity: #\n"); //Capacity: 11

    /* 使用字符串构建时, 如果字符串 Length > 16, 其 Capacity 同字符串的长度 */
    StringBuilder sb3 = new StringBuilder("abcdefghijklmnopqrstuvwxyz");
    str += sb3.Capacity.ToString("Capacity: #\n"); //Capacity: 26

    /* 指定 Capacity 和 MaxCapacity */
    StringBuilder sb4 = new StringBuilder(4, 10);
    str += sb4.Capacity.ToString("Capacity: #\t"); //Capacity: 4
    sb4.Append("1234567890");
    str += sb4.Capacity.ToString("Capacity: #\n"); //Capacity: 10
    try { sb4.Append("ABC"); }
    catch (Exception err) { str += err.Message + "\n"; } //容量小于当前大小...

    /* 用字符串构建, 同时指定 Capacity */
    StringBuilder sb5 = new StringBuilder("1234567890", 32);
    str += string.Format("Capacity: {0}\tLength: {1}\n", sb5.Capacity, sb5.Length); //Capacity: 32  Length: 10

    /* 从字符串截取构建, 同时指定 Capacity */
    StringBuilder sb6 = new StringBuilder("ABCDEFG", 1, 3, 12);
    str += string.Format("\"{0}\"\tCapacity: {1}\tLength: {2}", sb6, sb6.Capacity, sb6.Length); //"BCD"	Capacity: 12	Length: 3

    TextBox1.Text = str;
}

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练习:

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//ToString()
protected void Button1_Click(object sender, EventArgs e)
{
    StringBuilder sb = new StringBuilder("1234567890");
    string s1 = sb.ToString();     //1234567890
    string s2 = sb.ToString(2, 3); //345

    TextBox1.Text = s1 + "\n" + s2;
}

//Append()、AppendFormat()、AppendLine()
protected void Button2_Click(object sender, EventArgs e)
{
    StringBuilder sb = new StringBuilder(256);
    sb.Append("abcdefg");
    sb.AppendLine();
    sb.Append(3.1415926);
    sb.AppendLine();
    sb.AppendFormat("{0:#.##}", 3.1415926);
    TextBox1.Text = sb.ToString();
/*
abcdefg
3.1415926
3.14
*/
}

//Insert()、Remove()、Replace()
protected void Button3_Click(object sender, EventArgs e)
{
    StringBuilder sb = new StringBuilder("..........", 256);

    string s1 = sb.Insert(2, "ABC").ToString();  //..ABC........
    string s2 = sb.Insert(2, 3.14).ToString();   //..3.14ABC........
    string s3 = sb.Remove(6, 3).ToString();      //..3.14........
    string s4 = sb.Replace('.', '*').ToString(); //**3*14********

    TextBox1.Text = s1 + "\n" + s2 + "\n" + s3 + "\n" + s4;
}

//CopyTo
protected void Button4_Click(object sender, EventArgs e)
{
    string str = "..........";
    char[] cArr = str.ToCharArray();

    StringBuilder sb = new StringBuilder("1234567890");
    sb.CopyTo(2, cArr, 0, 3);
    TextBox1.Text = new string(cArr); //345.......
}

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